Coating substrates with reinforced resin matrices, such as liquid resins reinforced with fibers, glass microspheres, or other reinforcing materials, conventionally requires mixing the liquid resin with the reinforcing material and then painting or spraying the mixture onto the substrate, or dipping the substrate into the mixture. When only a portion of the substrate requires coating, accuracy and control requirements typically dictate the use of a spray coating process. Spray coating processes, however, are limited due to the low sprayability of the liquid resins which are typically highly viscous, the limit in attainable coating thickness, and the high amount of waste material generated.
Many liquid resins utilized in spray coating processes possess viscosities of about 20,000 centipoise (cps) or greater. At such high viscosities, pumping the liquid resin through the lines and nozzle of a spray coating apparatus is difficult and requires large amounts of energy. In order to reduce energy requirements and to simplify the spray coating process, the viscosity of the liquid resin is often reduced to about 2,000 cps by mixing the liquid resin with a solvent. Typically, however, solvents useful in spray coating processes are generally environmentally hazardous. Consequently, waste material from the spray coating process must be disposed of as hazardous waste.
Conventional spray coating processes comprise combining a liquid resin, solvents, reinforcing material, and other conventional constituents such as curing agents, biocides, etc., in a vat to form a mixture. This mixture is then pumped from the vat through lines to a nozzle where it is atomized and sprayed onto the substrate. Once the mixture has been applied to the substrate, the solvent is removed therefrom by the natural evolution of volatile gas and/or by applying heat to the mixture to hasten the solvent evolution.
During the solvent evolution, solvent near the substrate surface migrates to the coating surface, dragging liquid resin with it, and thereby forming resin starved areas in the coating. These resin starved areas result in poor adhesion between the coating and the substrate, and act as potential coating failure points. The effect of the solvent migration can be minimized by applying thinner coatings, less than about 0.04 inches, to the substrate. However, thick coatings of about 0.25 to about 0.50 inch or greater, are often required to attain the desired substrate protection, such as thermal protection.
An additional disadvantage of these coating processes is system clogging. Since all of the coating constituents are combined in a vat, they all must be pumped thorough the coating system as a single mixture. During the pumping, the liquid resin can begin to set up within the system, resulting in a clogged nozzle and/or lines. Furthermore, the reinforcement can accumulate within the lines or the nozzle, also causing clogging thereof.
What is needed in the art is an improved spray coating apparatus and process which reduces waste and system clogging while improving the structural integrity of thicker coatings.